Comments

As Zi Song has pointed out, direct induction is one of the easiest ways to go. You can also recognize that \[a_{n} = a_{n-1} \times \left(10^{2 \cdot 3^n} + 10^{3^n} + 1 \right) = a_{n-1} \times \left( \left(10^{3^n} \right)^2 + 10^{3^n} + 1\right)\]
It should be easy to show that \(3\) divides \(\left( \left(10^{3^n} \right)^2 + 10^{3^n} + 1\right)\). Hence, you can now conclude that \(3a_{n-1}\) divides \(a_n\).